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  1. #1

    polaris slt750 MFD fuse blows every time

    Hi.. my rebuilt ski has an issue.
    Original MFD was totally fried internally.
    Fried another one before I realised fuse was wrong..
    Now with the correct fuse it blows the fuse and replacement MFD is ok and works with new fuse..

    Every time I start engine this happens.
    Have checked ohms on flywheel coils.. have replaced LR regulator with new one, newer version with bilge supply ( not connected)
    I dont understand why this keeps occurring.. ski runs fine without the display but I would prefer it to work.
    Any ideas what could be going wrong here?
    Tia.


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by Mark-UK View Post
    Hi.. my rebuilt ski has an issue.
    Original MFD was totally fried internally.
    Fried another one before I realised fuse was wrong..
    Now with the correct fuse it blows the fuse and replacement MFD is ok and works with new fuse..

    Every time I start engine this happens.
    Have checked ohms on flywheel coils.. have replaced LR regulator with new one, newer version with bilge supply ( not connected)
    I dont understand why this keeps occurring.. ski runs fine without the display but I would prefer it to work.
    Any ideas what could be going wrong here?
    Tia.
    It is not clear to me what you are saying. Do you have a good working MFD? And the MFD works properly with the correct 1/4 Amp fuse installed?

    I am not understanding what the actual problem is

  3. #3
    Quote Originally Posted by K447 View Post
    Quote Originally Posted by Mark-UK View Post
    Hi.. my rebuilt ski has an issue.
    Original MFD was totally fried internally.
    Fried another one before I realised fuse was wrong..
    Now with the correct fuse it blows the fuse and replacement MFD is ok and works with new fuse..

    Every time I start engine this happens.
    Have checked ohms on flywheel coils.. have replaced LR regulator with new one, newer version with bilge supply ( not connected)
    I dont understand why this keeps occurring.. ski runs fine without the display but I would prefer it to work.
    Any ideas what could be going wrong here?
    Tia.
    It is not clear to me what you are saying. Do you have a good working MFD? And the MFD works properly with the correct 1/4 Amp fuse installed?

    I am not understanding what the actual problem is
    Yes i still have 2 MFD units, Round type, that are working. I bought every one i could find lol..
    So now, no matter which one i use, when the engine starts, i can see the display and even see revs.. but at some point it just cuts off, and i find the fuse has now blown. Replacing the fuse make the display work again.. Start the ski up.. and the display fuse will fail again, within a few seconds to maybe a minute.

  4. #4
    Click avatar for tech links/info, donation request K447's Avatar
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    Leave the MFD fuse out for this test.

    Connect voltmeter to the battery posts. Start engine and rev it up. Voltage should remain stable at about 14.x volts. If the voltage rises excessively then the charging system is not working properly.

  5. #5
    Quote Originally Posted by K447 View Post
    Leave the MFD fuse out for this test.

    Connect voltmeter to the battery posts. Start engine and rev it up. Voltage should remain stable at about 14.x volts. If the voltage rises excessively then the charging system is not working properly.
    I have found that my battery is dropping to 10.7 volts, its a new battery but it seems to me a Cell may be faulty.. im guessing this would probably have an affect on voltage regulation?

  6. #6
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by Mark-UK View Post
    Quote Originally Posted by K447 View Post
    Leave the MFD fuse out for this test.

    Connect voltmeter to the battery posts. Start engine and rev it up. Voltage should remain stable at about 14.x volts. If the voltage rises excessively then the charging system is not working properly.
    I have found that my battery is dropping to 10.7 volts, its a new battery but it seems to me a Cell may be faulty.. im guessing this would probably have an affect on voltage regulation?
    Voltage is dropping when the engine is running?

  7. #7
    Hi mate, the battery is dropping while left without a charge after a day.
    Just installed a new battery fully charged. Put meter probes on.. started up.. as soon as I rev the engine the voltage jumps up to around 30 volts.. not good.
    Incidentally I connected the spare orange bilge wire from the regulator to a meter and that showed nothing.
    This wire is spare, not connected to anything. My ski has no bilge pump.
    So for some reason I have an over volt fault..

  8. #8
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by Mark-UK View Post
    Hi mate, the battery is dropping while left without a charge after a day.
    Just installed a new battery fully charged. Put meter probes on.. started up.. as soon as I rev the engine the voltage jumps up to around 30 volts.. not good.

    Incidentally I connected the spare orange bilge wire from the regulator to a meter and that showed nothing.
    This wire is spare, not connected to anything. My ski has no bilge pump.
    So for some reason I have an over volt fault..
    The LR regulator exhibits several symptoms of failure.

    Replace it.

    More details can be found via my signature links, but there are several variations of the Polaris LR module that can be used in your machine.

    If you have been riding around with 30+ volts charging the battery, the battery itself may have been degraded. Excessive voltage means excessive charging current. Batteries can only tolerate so much excess charging before the degradation becomes significant.

    Even if it still charges and starts the engine. Battery lifespan may have been compromised.

    If the battery is a traditional lead-acid type (which. I do not use anymore) and it has removable cell caps, you may need to add distilled water to replace any water that has boiled off. Wear eye protection, gloves and old clothing when working with liquid acid batteries.

    Going forward, I recommend a sealed AGM battery, in particular the Deka ETX16.

  9. #9
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    Let's get back to the core issue, a constant blown MFD fuse.

    Why would the MFD fuse blow in an over volt condition? The fuses should be rated for 250v. Current draw from the MFD shouldn't increase, unless the over volt condition damaged the MFD internally causing an increase in draw exceeding the 1/4 amp fuse rating, but the MFD in question does in fact work, until the motor is started.

    Also interesting that a "NEW" LR module has also failed. Was it confirmed as operational at any point?

  10. #10
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by BlueFishCrisis View Post
    Let's get back to the core issue, a constant blown MFD fuse.

    Why would the MFD fuse blow in an over volt condition? The fuses should be rated for 250v. Current draw from the MFD shouldn't increase, unless the over volt condition damaged the MFD internally causing an increase in draw exceeding the 1/4 amp fuse rating, but the MFD in question does in fact work, until the motor is started.

    Also interesting that a "NEW" LR module has also failed. Was it confirmed as operational at any point?
    The voltage rating on a fuse is simply the maximum voltage the fuse can hold back after it has blown. Until the fuse blows the voltage rating does not matter much.

    The current (Amps) rating of the fuse matters a lot more.

    I have never tested how MFD respond to extreme over voltage. At some 'high enough' voltage level the electronics inside will begin to collapse and current flow will rise rapidly. My guess is that this is happening one side or the other of this 30 volts region, thereby blowing the 1/4 Amp fuse.

    If the fuse was rated for more Amps then the fuse may not blow so readily during this excessive voltage scenario and the additional current flow through the MFD just cooks it.

    I need to review the LR module replacement history in this particular case...

    Have we confirmed that the rest of the electrical system is actually good? Grounds are indeed solid, LR module connections actually where they should be?

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